Little holes in my lawn
Based on the timing and the extent of the damage, I suspect this is the result of vole activity. Voles don't hibernate. They tunnel and feed all winter long under the cover of snow. If the holes have a shallow flat bottom (not tunnel entrances), it may be the result of the feeding activity of squirrels or birds. In this case, it's likely that you would have seen the damage develop.
I'm attaching links to an article that describes some of the different critters that can cause hole damage. It was written for a warmer climate, but many of the culprits can be found here. I'm also attaching a link to an article with more information about voles.